Transforming load-controlled fatigue test data into a strain-based SN-curve: Results for unidirectional non-crimp glass fiber/epoxy composites tested in tension-tension

Abstract This study investigates fatigue in composite materials, focusing on challenges associated with testing fatigue properties using load-controlled experiments, and afterwards presenting results in terms of strain-based SN-curves. When simple elastic relations (Hooke’s law) are used to convert stress to strain, the strain-based fatigue lifetime may be underestimated. Motivated by experimental observations, an alternative procedure is used to establish a strain-based SN-curve based on the actual measured strains that specimens endure before failure in load-controlled fatigue tests. An SN-curve based on the fatigue equivalent strain is determined, which is claimed to represent strain-controlled fatigue better. The material system examined is a unidirectional non-crimp fabric glass fiber/epoxy composite, commonly used in wind turbine blade spar cap laminates. For this material, the equivalent SN-curve has an increase in the intersection constant by 58% and a decrease in the slope by 4% compared to the nominal SN-curve.